I would think that a tag alerting to the language at the start of the scene and then any to a language change should be sufficient once you've established that A and B are French and speak French but also English.

Okay, groovy! Thanks for responding. I'd rather the people reading the story choose the method they like best, because they're the ones who are going to be staring at the screen trying to sort it out. I already know who's speaking what language, after all.

I hate hitting passages of dialogue in a foreign language and having to go to the end of the chapter or story for translations!

I admire people who've taken the time to research the language and use it appropriately (presuming they have, and didn't just run it through Google translate), but the jumping up and down the page is very annoying.

I only have two very brief sections in actual French, one is basic French that I hope is made clear by the context used, and then the other one is a plot relevant pun that can't be done in English, so has to be told in French. And since the person to whom it's being told to also speaks French, it wouldn't make sense for the character to explain it, so I'm going to have to put a note somewhere.